The present application relates to semiconductor device manufacturing, and in particular, to in situ monitoring of deposition processes, such as LPCVD processes, using Raman spectroscopy.
Low-pressure chemical vapor deposition (LPCVD) is commonly used in fabrication of semiconductor devices for deposition of such materials as polysilicon, silicon dioxide (SiO2), silicon nitride (Si3N4), etc. For example, commercial LPCVD equipment is available to deposit polysilicon on wafers up to eight inches in diameter. Up to 200 wafers may be loaded in a typical LPCVD reactor or furnace, which is based on a hot-wall, resistance-heated, horizontal fused-silica tube design. The temperature of the wafers in the furnace is controlled by heating the LPCVD tube. Typical deposition conditions are temperatures from 580xc2x0 C. to 650xc2x0 C. and pressures from 100 to 400 mTorr. The usual source gas is 100% silane, which decomposes at the surface to leave silicon behind.
The structure of the deposited polysilicon is a function of the deposition conditions. For the usual LPCVD conditions (e.g., 100% silane source gas at about 200 mTorr), amorphous silicon films are deposited below 580xc2x0 C. and polycrystalline films are deposited above this temperature. As the deposition temperature increases in the polycrystalline range, the as deposited grain structure of the polysilicon films changes. At 600xc2x0 C., the grains are very fine; at 625xc2x0 C., the grains are well defined and can have a columnar structure perpendicular to the plane of the film. The grain size also tends to increase with film thickness. The crystal orientation of the polysilicon grains is dependent on the deposition temperature. For the standard LPCVD conditions (e.g., 100% silane source gas at 200 mTorr), the {110} orientation dominates between 600xc2x0 C. and 650xc2x0 C., whereas between 650xc2x0 C. and 700xc2x0 C., the {100} orientation dominates.
The electrical properties of polysilicon depend strongly on the grain structure of the film because the grain boundaries provide a potential barrier to the moving charge carriers and affect the conductivity of the films. The thermal conductivity of polysilicon is also a function of the grain structure of the deposited film. For fine grain films, the thermal conductivity is about 0.30-0.35 W/cm-K. This is 20-25% of the single-crystal value. For thicker films with larger grains, the thermal conductivity is between 50% and 85% of the single-crystal value.
Mechanical properties of polysilicon also depend on the deposition conditions. Thin films are generally under a state of stress, commonly referred to as residual stress. In semiconductor devices, the residual stress greatly affects the device performance. The as-deposited residual stress in the polysilicon film depends on the structure of the film. Polysilicon films are deposited in compressive stress the highest compressive stresses are in amorphous films, and polycrystalline films with a strong {110} texture. For films without a strong {110} texture, the stress tends to decrease with increasing temperature. Thicker films tend to have less stress.
As electrical and mechanical properties of films strongly depend on deposition conditions, it would be desirable to perform in situ monitoring of the thickness and composition of the film during the deposition process.
The present invention offers a novel method of in situ monitoring of a film being deposited on a wafer for manufacturing a semiconductor device. The method involves producing an incident beam of radiation directed during a deposition process to a film being deposited on a wafer in a deposition reactor. The Raman scattered radiation resulted from interaction of the incident beam with molecules of the deposited film is detected to produce a Raman spectrum of the deposited film.
In accordance with a preferable embodiment of the invention, a parameter of the deposited film determined based on the Raman spectrum may be compared with a predetermined range, to adjust the deposition process if the parameter is outside the predetermined range, which may be preprogrammed for a particular film to be deposited.
For example, the thickness of the deposited film determined based on the Raman spectrum may be compared with a predetermined thickness value to stop the deposition process if the thickness reaches the predetermined thickness. Also, crystal grain size of the deposited film determined based on the Raman spectrum may be compared with a predetermined size to increase the deposition temperature if the crystal grain size is less than the predetermined size. Further, the crystal orientation of the deposited film may be determined based on the Raman spectrum. If the crystal orientation is not correct, the deposition temperature may be adjusted to correct the crystal orientation.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.